Method of removing residual stress for improving fatigue strength of boundary of heated zone

ABSTRACT

A method for removal of residual tensile stresses from a locally casehardened steel object is disclosed. In accordance with the method, a projecting fillet is provided at the boundary of the zone on the object to be hardened. The fillet is removed after hardening is accomplished.

United States Patent Tomita et al.

[451 Mar. 28, 1972 METHOD OF REMOVING RESIDUAL STRESS FOR IMPROVING FATIGUE STRENGTH OF BOUNDARY OF HEATED ZONE Inventors: Katsunobu Tomita; Kentaro Ishii, both of Tokyo; Yoshito Tanaka, Nara; Takao Saito, Nishinomiya, all ofijapan Assignees: Japanese National Railways, Tokyo; Su-

mitomo Metal Industries Limited, Osaka J apan Filed: Sept. 17, 1969 Appl. No.: 858,814

Foreign Application Priority Data Sept. 18, 1968 Japan ..43/67744 [1.8. CI ..l48/134, 148/145, 148/146, 148/150, 148/151, 148/154 [51] Int. Cl. .C2ld l/06,C21d l/08, C2ld1/10 [58] Field oISearch ..148/l34, 145, 146, 149, 150, 1481154, 15l

References Cited UNITED STATES PATENTS 2,086,801 7/1937 Hayden ..148/l49 2,801,193 7/1957 Parker et a1. ..148/149 Primary Examiner-L. Dewayne Rutledge Assistant Examiner-W. W. Stallard Attorney-Watson, Cole, Grindle & Watson [5 7] ABSTRACT A method for removal of residual tensile stresses from a locally casehardened steel object is disclosed. In accordance with the method, a projecting fillet is provided at the boundary of the zone on the object to be hardened. The fillet is removed after hardening is accomplished.

4 Claims, 3 Drawing Figures PATENTEMms m2 3.652.345

SHEET 1 [IF 2 Fig: l M Y 5 3 E FE5/0U4Z INVENTOR ATTORNEY PATENTEDWZMBYZ 3,652,345

SHEET 2 [IF 2 INVENTOR KM W 41h The instant invention relates to a method for removing residual stresses from hardened steel objects to improve the fatigue strength thereof at the boundary of the hardened zone thereon. The object may be, for example, a straight or curved shaft for a railway car axle, an armature shaft, a mill roll, a crank shaft, or the like.

Generally, steel objects are casehardened to improve their resistance to fatigue and wear. However, when steel objects are locally casehardened, it is not unusual for the fatigue strength of the object to be reduced at the boundaries of the hardened zone. The tendency of the steel object to break increases correspondingly with a reduction in fatigue strength.

Accordingly, a very important object of the present invention is to provide a method for erasing residual stresses generated at the boundaries of a hardened zone on a casehardened steel object.

It has been discovered in accordance with the present invention, that the residual tensile stresses which are generated at the boundaries of a hardened zone and which have an adverse influence on the fatigue strength of a hardened object do not penetrate into the object very deeply and are generally limited to the surface layers of the object which was subjected to the thermal influence of the induction-hardening or flamehardening operation. The present invention advantageously utilizes this phenomenon and is effective to erase residual tensile stresses by providing a projecting fillet at locations on an object where residual tensile stresses will be developed during casehardening by an induction-hardening or flame-hardening operation. The residual tensile stresses are removed by removing the fillet after hardening has been accomplished.

In the accompanying drawings:

FIG. 1 is a graph illustrating the relationship between the position of a conventional induction coil and the distribution of residual tensile stresses on the surface of a steel object which has been locally hardened by a conventional inductionhardening method.

P16. 2 is an elevational, longitudinal view, partly in cross section, showing an embodiment of the method of the present invention.

FIG. 3 is a graph comparing the distribution of residual stresses on the surface of a shaft hardened by the conventional method and residual stresses on the surface of a shaft hardened and relieved by the method of the present invention.

Before explaining the present invention, a conventional method shall be described with reference to FIG. 1 wherein the line Y in the graph illustrates the distribution of residual stresses in a longitudinal direction along the surface of a conventionally hardened, straight, cylindrical steel shaft. In FIG. 1, X refers to an induction coil and Z refers to a hardened zone on the surface of the steel object (shown in dashed lines). The reference letters X, Y, and Z in FIG. 1 are all shown as being disposed at corresponding axial positions. As is understood from HQ 1, at locations within the hardened zone Z, large residual compressive stresses are developed. However, at the areas adjacent the boundaries of the hardened zone, the character of the residual stresses is reversed and residual tensile stresses are developed. These residual tensile stresses are quickly reduced after passing a maximum. Thus, in the case of steel objects locally hardened by a conventional casehardening operation, residual compressive stresses will be developed in the hardened zone itself. However, at areas adjacent the boundaries of the hardened zone, residual tensile stresses will be developed causing a reduction in the fatigue strength of the object and an increase of its susceptibility to breakage at such locations.

The present invention is explained below with reference to a locally induction-hardened shaft. As can be seen in FIG. 2, projecting fillets 2, each having a width sufficient to span areas in which residual tensile stresses are generated and a height corresponding at least to the depth to which such residual tensile stresses penetrate, are provided on the outer periphery of a shaft 1 at respective ends of the zone thereof to be hardened. Shaft 1 is then hardened by an otherwise conventional induction-hardening method with an induction coil 3. Thereafter, projecting fillets 2 are removed by mechanical working to finish the shaft. In view of the removal of the projecting fillets 2, the residual tensile stresses which would otherwise reduce the fatigue strength of the object, are not present.

FIG. 3 graphically depicts the results of an X-ray examination and measurement of the residual stresses on the surface of a hardened object and plots the intensity of such stresses against distance along the axis of the shaft in both the case of a shaft locally casehardened by the above-mentioned method of the present invention and the case of a shaft locally casehardened by a conventional method in which projecting fillets 2 were not provided onthe shaft. The letter A represents a curve plotted from the results of an analysis of a shaft hardened by a conventional method and the letter B represents a curve plotted from the results of an analysis of a shaft hardened by the method of the present invention. The test shafts were constructed of a carbon steel (A181 1038) containing 0.38 percent carbon. The diameter of the zone to be hardened was 190 millimeters. The hardening conditions were such that the zone required to be hardened, which had a dimension designated by the letter a, was heated at a casehardening temperature of 830 C. for a heating time of 120 seconds utilizing a hollow induction coil made of copper. The cross sectional dimensions of the coil, which had an inside diameter of 225 millimeters, were 20 millimeters deep by 12 millimeters wide. After heating, the shaft was quenched. The shaft to be hardened by the method of the present invention was provided with projecting fillets 2 which had a height of 3 millimeters (diameter of 196 millimeters) and a width of millimeters.

As is evident from FIG. 3, the curve A, which represents the conventional method, illustrates that very large residual tensile stresses were developed adjacent the boundaries of the hardened zone. However, the curve B, representing the method of the present invention, illustrates that no residual tensile stresses were present at such locations. Therefore, the method of the present invention is very effective to reduce residual tensile stresses and thereby operates to improve the fatigue strength at the boundaries of a hardened zone on a locally casehardened steel object.

Since the intensity of the residual tensile stresses generated at locations adjacent the boundaries of the hardened zone is related to the depth of hardening, it will be obvious to those skilled in the art that the shape and dimensions of the projecting fillet will vary depending upon the hardening conditions to be employed and should be detennined in advance by empirical methods.

What is claimed is:

l. A method of removing residual stresses in a locally case hardened steel object to improve the fatigue strength of the object at the boundary of the hardened zone comprising providing a projecting fillet at the boundaries of the zone of the steel object to be hardened prior to said case hardening and then removing said fillet after hardening to thereby erase the residual tensile stresses.

2. A method according to claim 1 wherein the projecting fillet is disposed in an area wherein residual tensile stresses in the axial direction are generated on the surface of the steel material.

3. A method according to claim 1 wherein the steel material is locally casehardened by induction-hardening or flamehardening.

4. A method according to claim 3 wherein the steel material is a straight or curved cylindrical shaft for a railway car axle, armature shaft, mill roll, or crank shaft. 

2. A method according to claim 1 wherein the projecting fillet is disposed in an area wherein residual tensile stresses in the axial direction are generated on the surface of the steel material.
 3. A method according to claim 1 wherein the steel material is locally casehardened by induction-hardening or flame-hardening.
 4. A method according to claim 3 wherein the steel material is a straight or curved cylindrical shaft for a railway car axle, armature shaft, mill roll, or crank shaft. 